Histologic and ultrastructural studies demonstrated widespread abnormalities in all connective tissue components of floppy mitral valve. Alterations in collagen and accumulations of proteoglycans were considered to be nonspecific changes that may be secondary to the abnormal mechanical forces to which floppy mitral valves are subjected because of their excessively large surface area. The presence of excessive amounts of proteoglycans may interfere with the normal assembly of collagen and elastic fibers. Abnormalities of elastic fibers resemble those in other conditions characterized by structural dilatation or tissue expansion. The alterations in elastin could result from defective formation, increased degradation, or both.